Differential pressure sensors measure a difference in pressure between two points of measurement (e.g. P1 and P2) of a fluid. A differential pressure sensor (or transducer) converts the pressure difference to an electrical signal that can be measured to determine the differential pressure. For example, a differential pressure sensor may be used in an oil pipe to measure the pressure before and after an orifice in the fuel pipe, from which the flow rate of the oil can be determined. Such devices are typically manufactured using micro-machined or Micro-Electro-Mechanical System (MEMS) based techniques. One common technique for manufacturing a pressure sensor is to attach a MEMS device onto a substrate, such as a ceramic or printed circuit board (PCB) substrate, along with etching and bonding techniques to fabricate very small, inexpensive devices.
The pressure-sensing die may typically be formed from a semiconductor material such as silicon. FIG. 1 is a sectional view of a MEMS type pressure sensing device or die 100 of the prior art. Pressure sensing device 100 may be formed from a silicon wafer by methods such as dicing to produce a silicon substrate structure 101. Structure 101 is thinned to create a chamber 105 and a thinned portion defining a diaphragm 103. Semiconductor substrate 101 may be thinned by any suitable means. For example, substrate 101 may be thinned using anisotropic etching as known in the art. Resistive elements 107 are formed on the surface of the diaphragm 103. Resistive elements 107 exhibit resistance that is proportional to the strain placed on the thinned semiconductor material forming diaphragm 103.
FIG. 2 is an illustration of a conventional MEMS differential pressure sensor 200 using pressure sensing device 100. Pressure sensing device 100 may be mounted to a support structure 201 which is, in turn bonded to a base plate 203, which may be formed from a non-corroding material, for example, stainless steel. Sensing device 100 and support structure 201 may be bonded to base plate 203, which may also be termed a header, by an adhesive (not shown). Support structure 201 is used to isolate pressure sensing device 100 from sources of strain that are unrelated to pressure, such as thermal expansion, which varies between pressure sensing device 100 and base plate 203. An opening 221 is defined in base plate 203 defining an aperture, which is in gas or fluid communication with the underside of the diaphragm of pressure sensing device 100. While there is some difference between the thermal expansion coefficient of pressure sensing device 100 and the thermal expansion coefficient of support structure 201, support structure 201 may be formed from glass or similar material which has a coefficient of thermal expansion closer to that of silicon pressure sensing device 100 as compared to the coefficient of thermal expansion of the stainless steel making up base plate 203. This reduces, but does not eliminate non-pressure related errors measured by pressure sensing device 100 due for example, to stress exerted on pressure sensing device 100 due to the differences in thermal expansion between pressure sensing device 100 and support structure 201. If pressure sensing device 100 is bonded directly to base plate 203, these non-pressure related errors may be even greater.
Pressure sensor 200 includes an upper housing 220. Upper housing 220 is configured to provide a sealed attachment to base plate 203. An enclosed volume 217 is defined between upper housing 220 and base plate 203. A flexible corrugated diaphragm 215 serves to divide enclosed volume 217 into a first volume 217 and a second volume 213. A port 219 is defined through a wall of upper housing 220 and is in communication with a second section or portion of gas or fluid whose pressure P2 is to be measured, and which comes in contact with another side of the pressure sensing device 100 adjacent first volume 217. Pressure sensing device 100 further includes electrical components which create and transmit an electrical signal indicative of a pressure exerted on device 100. In applications where the fluid being tested is a harsh medium, such as fuel or oil, harsh media may corrode the electrical components of device 100. In such embodiments, isolation of device 100 from the fluid being tested is accomplished by flexible corrugated diaphragm 215. An oil fill port 209 is provided through base plate 203. Oil fill port 209 allows volume 213 between device 100 and flexible diaphragm 215 to be filled with a non-corrosive fluid such as silicone oil. When the cavity defining volume 213 is filled, oil fill port 209 is sealed, for example, by welding a ball 211 across the opening of oil fill port 209. The oil in volume 213 is thus fully enclosed and in fluid communication with the upper surface of device 100.
Port 219 may be threaded to allow pressure sensor 200 to be attached via a fitting to a line or other transmission means in communication with the gas or fluid to be tested or measured. The gas or fluid being measured enters port 219 and fills interior volume 217. When interior volume 217 is filled, the fluid being measured is in contact with the upper side of flexible diaphragm 215. Pressure exerted by the gas or fluid being measured is transmitted through flexible diaphragm 215 to enclosed volume 213 of oil. The force applied to the oil by flexible diaphragm 215 is transmitted throughout the oil and to the surfaces containing the oil, including the upper surface of pressure sensing device 100.
When pressures P1 and P2 are exerted on pressure sensing device 100, an electrical signal through piezoresistive elements (107 shown in FIG. 1), formed in the upper surface of the diaphragm of pressure sensing device 100, varies responsive to variations in the piezoresistive elements. The electrical signal is representative of the differential force applied to the surface of pressure sensing device 100. The electrical signal is conducted via bond wires 202 to conductive pins 205 which may be electrically connected to other system circuitry via an electrical conductor, such as a control circuit, or converted to pressure data which may be stored, by way of non-limiting example, in an electronic memory.
Flexible diaphragm 215 and oil filled volume 213 isolate pressure sensing device 100, bond wires 202 and conductive pins 205 from the corrosive or harsh media being measured via port 219. Additionally, volume 213 containing the oil must be sealed such that leakage or contamination of the oil within volume 213 does not occur. Conductive pins 205 carrying the electrical signal from pressure sensing device 100 must pass through base plate 203 to allow external connection of other system components. Conductive pins 205 are enclosed in a glass or ceramic material fired into a tube or opening 207 which forms a hermetic seal with base plate 203. Hermetic seals are expensive to produce and are fragile, but are necessary to ensure the integrity of volume 213.
Pressure sensors, such as the sensor of FIG. 2, are intended to respond only to pressure changes of the fluid being tested. However, due at least in part to design and manufacturing constraints, additional stimuli are introduced which cause changes in the pressure sensor output which are not related to pressure. For example, stimuli such as stress, temperature, leakage current within the device, vibrations, and the like may cause the output of the sensor to change without relation to pressure. These non-pressure related changes introduce errors to the pressure reading of the sensor. Sensing systems which reduce the effects of non-pressure related stimuli experienced by a pressure sensor are desired.